I agree with a lot of the other posts esspecially Crabbys. Here are my two cents:
I for sure have back pain when I am flaring. I know part of it is the Remicade which causes me all kinds of body aches (but is still way worth it), some of it is the disease itself, but it is also due to how much time I spend lying or even sitting down when I am sick. Lying in bed all day may be unavoidable at times, but I know it is not good for my back.
I know sit ups are hard for some Crohnies, but if you can get into a habit of doing them daily, I believe it is the number one thing that strengthens my back. My husband who works a very physical job also sees an increase in back pain when he doesn’t do his crunches for a few days. We do them morning and night, and it has really helped us both. Even just doing a few each day and building up would probably help some.
Hope this idea helps you, or you find something else that can. I know we are all different, but like I said, this really works for me. Good luck!